1. Field of the Invention
The present invention relates to a wireless communication system, and to a method for constructing a random access channel (RACH) and a method for transmitting a RACH signal.
2. Discussion of the Related Art
A method for constructing a random access channel (RACH) and a method for transmitting a RACH signal will hereinafter be described in detail.
There is proposed a method for preventing the occurrence of an inter-symbol interference (ISI) while each symbol of the sub-carrier is transmitted over a multi-path channel in either an Orthogonal Frequency Division Multiplexing (OFDM) or a Discrete Multi-Tone (DMT), and a detailed description of the above method will hereinafter be described.
For example, a guard time (GT) longer than a maximum delay spread of a channel is inserted between successive symbols. In brief, this guard time (GT) is called a cyclic extension. The cyclic extension (CE) is classified into a cyclic prefix (CP) and a cyclic suffix (CS). The cyclic prefix (CP) copies the signal of the last time of an effective symbol time, and inserts the copied signal to the front part, such that it prevents orthogonality between sub-carriers from being broken or damaged. The cyclic suffix (CS) copies the signal of a first time of an effective symbol, and attaches the copied signal to the end part.
A single RACH signal may include a CP time, a sequence time, and a guard time. The CP time copies a signal of the last time of the effective symbol time, and inserts the copied signal to the front part, such that it prevents orthogonality between sub-carriers caused by the signals received with different time delays.
The sequence time corresponds to the effective symbol time of the RACH signal. In fact, the sequence time indicates a time interval in which the sequence defined by the RACH signal is actually transmitted. In more detail, the sequence time is a time occupied by an actual transmission end after the defined sequence has executed necessary steps (e.g., over_sampling). The guard time (GT) of the RACH signal is indicative of an empty space having no transmission (Tx) signal, such that it prevents the next-symbol orthogonality from being broken or interfered by the signals received with relatively-high time delays.
A general RACH transmission structure and the influence of a frequency offset will hereinafter be described in detail.
A random access channel (RACH) is a channel used when a user equipment (UE) acquires initial uplink synchronization. If the user equipment (UE) is firstly powered on, or the terminal (UE) is switched to an active status after it has been in an idle mode for a long period of time, uplink synchronization should be re-established. The RACH is generally adapted to re-establish the uplink synchronization, and need not establish time synchronization or frequency synchronization.
The RACH basically supports a multi-user mode. Each user equipment (UE) transmits a specific preamble sequence when accessing the RACH. If a base station (BS) recognizes the preamble sequence received from the user equipment (UE), it transmits the recognized preamble sequence to a downlink. The user equipment (UE) updates its own time synchronization information using specific information contained in a downlink signal received from the base station (BS). In this case, if the base station (BS) transmits frequency synchronization information along with the time synchronization information, this frequency synchronization information can also be used to correct the user equipment (UE).
The basic requirements for RACH are used to satisfy a round trip delay (RTD) and the condition of a path loss, irrespective of a UE speed, a frequency offset, a cell radius, etc.
For example, the 3GPP LTE communication standard serving as one of current communication system-associated standards has been intensively discussed about the RACH structure which has the length of a 1 ms TTI (transmission Timing Interval), a cyclic prefix (CP) of 102.6 μs, a sequence length of 0.8 ms, a guard time (GT) of 97.5 μs. In this case, the time length other than the channel delay spread in the cyclic prefix (CP) has been intensively discussed as the length of the guard time (GT).
The cell radius of about 15 km can be used as the above-mentioned CP or GT.
In this case, the CP and GT may have different lengths requisite for individual cell radiuses. However, the cell size may be larger than 15 km as necessary. Presently, many developers are conducting an intensive research into a method for covering the length of more than 100 km, such that an additional RACH structure is needed.